1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine, an electrostatic printing machine, a printer, a facsimile and an electrostatic recording machine, an image forming method, and a process cartridge.
2. Description of the Related Art
Various known methods have hitherto been used for formation of an electrophotographic image. In general, the surface of a latent electrostatic image bearing member (hereinafter sometimes referred to as a “photoconductor,” an “electrophotoconductor” or an “image bearing member”) is charged and the charged surface is then exposed to form a latent electrostatic image thereon. Subsequently, the latent electrostatic image is developed with a toner to form a visualized image on the latent electrostatic image bearing member. The visualized image thus formed is transferred onto a recording medium directly or through an intermediate transfer member and the visualized image thus transferred is fixed to the medium by application of heat and/or pressure to obtain a record in which the image is formed on the recording medium. The toner particles left on the latent electrostatic image bearing member after transferring the visualized image are then removed with a known method that uses a blade, a brush, a roller or the like.
As a full color image forming apparatus which utilizes such an electrophotographic system, two systems are commonly known. One system is referred to as a single system (or a single drum system) in which an image forming apparatus is equipped with one latent electrostatic image bearing member and is also equipped with 4 developing units corresponding to fours colors such as cyan, magenta, yellow and black colors. In such a single system, visualized images of four colors are formed on a latent electrostatic image bearing member or a recording medium. In this single system, a charging unit, an exposing unit, a transferring unit and a cleaning unit that are arranged around the latent electrostatic image bearing member can be integrated and can be designed with small size at low cost as compared with a tandem system described hereinafter.
The other system is a system referred to as a tandem system (or a tandem drum system) in which an image forming apparatus is equipped with a plurality of latent electrostatic image bearing members (see Japanese Patent Application Laid-Open (JP-A) No. 05-341617). Commonly, for one latent electrostatic image bearing member, a charging unit, a developing unit, a transferring unit and a cleaning unit are arranged one by one to form one image forming element, and the image forming apparatus is equipped with plural (commonly, four) image forming elements. In this tandem system, a monocolor visualized image is formed by one image forming element and the visualized image is sequentially transferred onto a recording medium to from a full color image. In this tandem system, since each colored visualized image can be formed by parallel processing, an image can be formed at a high speed. That is, the tandem system requires a time for an image formation treatment which is about ¼times shorter than that in case of the single system, and also can cope with four-times high-speed printing. Also, it is possible to substantially enhance durability of each unit in an image forming element, including a latent electrostatic image bearing member. The reason is as follows. That is, in the single system, charging, exposing, developing and transferring steps are performed 4 times by one latent electrostatic image bearing member to form one full color image, whereas, in the tandem system, an operation of each step can be performed only one time by one latent electrostatic image bearing member.
However, the tandem system has such a problem that plural image forming elements are arranged and therefore the size of the entire image forming apparatus increases, resulting in high cost.
The above problem is solved by decreasing the diameter of the latent electrostatic image bearing member, down-sizing of each unit arranged around the latent electrostatic image bearing member and down-sizing of one image forming element. As a result, not only the effect of down-sizing of the mage forming apparatus, but also the effect of reducing the material cost can be exerted, and thus entire cost reduction could be attained to some degree. However, with the progress in down-sizing of the image forming apparatus, there arises such a new problem that it is required to impart high performances to each unit with which the image forming element is equipped, and to remarkably enhance stability.
Recently, market's requirements such as energy-saving and speeding-up on image forming apparatuses such as printer, copying machine and facsimile have become stronger. To achieve good performances, it is important to improve thermal efficiency of a fixing unit in the image forming apparatus.
Commonly, in the image forming apparatus, an unfixed toner image is formed on a recording medium such as recording sheet, printing paper, photographic paper or electrostatic recording paper by an image forming process such as electrophotographic recording, electrostatic recording or magnetic recording processes using an indirect transferring system or a direct transferring system. As a fixing unit configured to fix the unfixed toner image, for example, contact heating systems such as heating roller system, film heating system and electromagnetic induction heating system are widely employed.
The fixing unit of heating roller system has such a basic configuration comprising a heat source such as halogen lamp inside, a fixing roller whose temperature is controlled to a predetermined temperature, and a pair of rotary rollers with a pressurizing roller to be pressure-contacted with the fixing roller. A recording medium is inserted into a contact portion (so-called a nipping section) of the pair of rotary rollers and transported, and then the unfixed toner image is melted and fixed by heat and pressure from the fixing roller and the pressurizing roller.
The fixing unit of the film heating system is proposed for instance in JP-A Nos. 63-313182 and 01-263679. Such a fixing unit of the film heating system makes a heating element supported fixedly to a supporting member and a recording medium come closely contact through a thin fixing film having heat resistance, and makes the fixing film to slide to a heating element, thereby feeding heat of the heating element to the recording medium through the fixing film while moving the heating element.
As the heating element, for example, it is possible to use a ceramic heater comprising a ceramic substrate made of alumina or aluminum nitride having properties such as heat resistance, insulating properties and good thermal conductivity, and a resistive layer formed on the ceramic substrate. In such a fixing unit, a thin fixing film having low heat capacity can be used and the fixing unit has higher heat transfer efficiency than that of the fixing unit of heating roller system, and thus the duration of warm-up period can be shortened and quick-start and energy-saving can be realized.
As the fixing unit of an electromagnetic induction heating system, for example, there is proposed a technology in which Joule heat is generated by an eddy current generated in a magnetic metallic member through a magnetic alternating field and a heating element including a metallic member is allowed to cause electromagnetic induction heat generation (see JP-A No. 08-22206).
In such a fixing unit of the electromagnetic induction heating system, since the visualized image is uniformly melted with heating in a state of being sufficiently covered, a film comprising a rubber elastic layer on the surface is formed between a heating element and a recording medium. When the rubber elastic layer is formed of a silicone rubber, thermal responsiveness deteriorates because of low thermal conductivity, and thus a temperature difference between the internal surface of the film to be heated from the heating element and the external surface of the film in contact with the toner. When the amount of the toner adhered is large, the surface temperature of the belt quickly decreases and fixation performances can not be sufficiently secured, and thus so-called cold offset may occur.
In the fixing unit of the electrophotographic image forming apparatus, releasabiliy (hereinafter sometimes referred to as an “anti-offset properties”) of the toner to the heating member are required. The anti-offset properties can be improved by the presence of a releasing agent on the surface of the toner. When the toner other than a predetermined toner is used or the toner is reused, the amount of the releasing agent, which is present on the surface of the toner, decreases and anti-offset properties may deteriorate.
With the development of the electrophotographic technology, a toner having excellent low-temperature fixation properties, anti-offset properties and storage stability (blocking resistance) is required and, for example, there are proposed a toner containing a linear polyester resin having defined physical properties such as molecular weight (see JP-A No. 2004-245854), toner containing a non-linear crosslinking type polyester resin using rosins as an acid component in a polyester (see JP-A No. 04-70765), a toner having fixation properties improved by using a resin modified with maleic acid (see JP-A No. 04-307557) and a toner containing a mixture of a low molecular weight resin and a high molecular weight resin (see JP-A No. 02-82267).
It has been found that a conventional binder resin does not sufficiently meet the market's requirements as current image forming apparatus becomes faster and energy-saving. It becomes very difficult to maintain sufficient fixation properties with the reduction of the fixation time in a fixing step and the decrease of the heating temperature by means of a fixing unit. When a low molecular weight resin is used as the binder resin, there arises a problem that a toner is aggregated during storage because a glass transition temperature necessarily decreases, thus resulting in poor storage stability. As current image forming apparatus become faster, a reduction in image quality becomes remarkable particularly in high-speed continuous printing because of poor electrification of toner and toner filming which is caused due to poor dispersion of internal additive.
Furthermore, rosins used in JP-A Nos. 04-70765 and 04-307557 are effective for improvement of low-temperature fixation properties, but have a drawback that odor is likely to occur depending on the kind of rosins.
Therefore, it is now required to quickly provide an image forming apparatus, an image forming method and a process cartridge, which are excellent in low-temperature fixation properties, anti-offset properties, storage stability, rising property of electrification and filming resistance, which can also reduce generation of odor, and which are capable of forming high-quality images for a long period of time.